Shooting long fiber spans with high loss fiber

ABSTRACT

A method of tracing a complete span of a fiber includes inferring a reference point on the fiber based on a measured length of the fiber, setting a pulse width and a measurement range based on the inferred reference point, shooting a fiber from a master unit, attached to one end of the fiber, to collect trace past the inferred reference point, shooting a fiber from a slave unit, attached to an opposite end of the fiber, to collect trace past the inferred reference point, cropping the collected traces captured by the master unit and the slave unit past the inferred reference point, inverting a slope of the trace of the slave unit using measurement loss information and combining the trace of the master unit and the trace of the slave unit, with the inverted slope, to obtain a complete and accurate trace of the fiber.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation Application of U.S. patentapplication Ser. No. 15/115,023, filed Jul. 28, 2016, which is aNational Stage Application of PCT/US2015/013519, filed on Jan. 29, 2015,which is based upon and claims the benefit of priority from U.S.Provisional Patent Application No. 61/934,174, filed Jan. 31, 2014, thedisclosures of all of which are incorporated by reference herein intheir entireties.

BACKGROUND 1. Field

The invention is related to event detection and mapping across thecomplete span of a fiber, and more particularly to measurement of thecomplete span distance of a fiber, measurement of complete loss acrossthe span of the fiber, event detection and mapping across the completespan of a fiber and achieving trace showing the complete span of thefiber.

2. Related Art

The background information provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventor, to the extent it is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

Long spans of fiber—on the order of 200 km—with splices every 5 km madeup of fiber with 0.3 dB/km loss and 0.1 dB per-splice loss drives theneed for over 64 dB of dynamic range. To detect events on such aspan—across the complete span an optical time-domain reflectometer(OTDR) would require 70 dB of dynamic range. This is far beyond thecapability of products in the field today.

In the current optical technology, no OTDR can shoot the complete spanand measure the length of the fiber described above as it would requireover 64 dB of dynamic range.

While high loss spans can be difficult to shoot, long spans in generalare difficult to shoot for the same reasons as described above. WhileOTDRs can shoot 200 Kilometers (km) low loss spans, the eventspacing/resolution can be poor due to the wide pulse widths required todo so. Accordingly, using shorter pulse widths on some long spans forbetter event location/resolution. Use of a 20 uS pulse width gives up toa 2 km location uncertainty as well as burying any events spaced closerthan 2 km apart.

Therefore, there is a need for the ability to shoot a long fiber spanfor measurement of complete span distance, which cannot be simplyestimated, measurement of complete loss across span, event detection andmapping across complete span and for acquiring a trace showing completespan of the fiber, so that the customer has confidence the fiber has notbe tampered with or modified.

SUMMARY

Exemplary implementations of the present invention address at least theabove problems and/or disadvantages and other disadvantages notdescribed above. Also, the present invention is not required to overcomethe disadvantages described above, and an exemplary implementation ofthe present invention may not overcome any of the problems listed above.

According to an aspect of an exemplary embodiment, a method of tracing acomplete span of a fiber includes inferring a reference point on thefiber based on a measured length of the fiber, setting a pulse width anda measurement range based on the inferred reference point, shooting afiber from a master unit, attached to one end of the fiber, to collecttrace past the inferred reference point, shooting a fiber from a staveunit, attached to an opposite end of the fiber, to collect trace pastthe inferred reference point, cropping the collected traces captured bythe master unit and the slave unit past the inferred reference point,inverting a slope of the trace of the slave unit using measurement lossinformation, and combining the trace of the master unit and the trace ofthe Slave Unit, with the inverted slope, to obtain a complete andaccurate trace of the fiber.

According to another exemplary embodiment, the pulse width and themeasurement range are set to be greater than half the measured length ofthe fiber.

According to an aspect of another exemplary embodiment, a non-transitorycomputer readable recording medium storing a program used in anapparatus, including at least one processor, for tracing a complete spanof a fiber, causes said at least one processor to infer a referencepoint on the fiber based on a measured length of the fiber, set a pulsewidth and a measurement range based on the inferred reference point,shoot a fiber from a master unit, attached to one end of the fiber, tocollect trace past the inferred reference point, shoot a fiber from aslave unit, attached to an opposite end of the fiber, to collect tracepast the inferred reference point, crop the collected traces captured bythe roaster unit and the slave unit past the inferred reference point,invert a slope of the trace of the slave unit using measurement lossinformation, and combine the trace of the master unit and the trace ofthe slave unit, with the inverted slope, to obtain a complete andaccurate trace of the fiber.

According to another exemplary embodiment, the program further causesthe said at least one processor to set the pulse width and themeasurement range to be greater than half the measured length of thefiber.

According to an as of another exemplary embodiment, an apparatus fortracing a complete span of a fiber the apparatus includes at least onememory operable to store program code, at least one processor operableto read the program code and operate as instructed by the program code,the program code including inferring code configured to cause the atleast one processor to infer a reference point on the fiber based on ameasured length of the fiber, setting code configured to cause the atleast one processor to set a pulse width and a measurement range basedon the inferred reference point, first shooting code configured to causethe at least one processor to shoot a fiber from a master unit, attachedto one end of the fiber, to collect trace past the interred referencepoint, second shooting code configured to cause the at least oneprocessor to shoot a fiber from a slave unit, attached to an oppositeend of the fiber, to collect trace past the inferred reference point,cropping code configured to cause the at least one processor to crop thecollected traces captured by the master unit and the slave unit past theinferred reference point, inverting code configured to cause the atleast one processor to invert a slope of the trace of the slave unitusing measurement loss information, and combining code configured tocause the at least one processor to combine the trace of the master unitand the trace of the slave unit, with the inverted slope, to obtain acomplete and accurate trace of the fiber.

According to another exemplary embodiment, the setting code is furtherconfigured to cause the at least one processor to set the pulse widthand the measurement range to be greater than half the measured length ofthe fiber.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates a fiber with a master and slave connected to oppositeends to determine the length of the fiber, according to an exemplaryembodiment.

FIG. 2 illustrates a fiber going through a process of tracing a completespan and event detection, according to an exemplary embodiment.

FIG. 3 is a flowchart describing a method of determining the length of afiber currently used by other OTDRs available in the market, accordingto another exemplary embodiment.

FIG. 4 is a flowchart describing a method of tracing a complete span andevent detection in a fiber, according to another exemplary embodiment.

FIG. 5 illustrates a fiber with a master and slave connected to oppositeends to determine the length of the fiber while incorporating the errorsin the time of flights of light pulses, according to an exemplaryembodiment.

FIG. 6 illustrates a functional block diagram of an embodiment of anapparatus which determines a length of a fiber, traces a complete spanof a fiber, and performs event detection in a fiber, according to anexemplary embodiment.

DETAILED DESCRIPTION

The following detailed description is provided to assist the reader ingaining a comprehensive understanding of the methods, apparatuses and/orsystems described herein. Various changes, modifications, andequivalents of the systems, apparatuses and/or methods described hereinwill suggest themselves to those of ordinary skill in the art.Descriptions of well-known functions and structures are omitted toenhance clarity and conciseness.

The terms used in the description are intended to describe embodimentsonly, and shall by no means be restrictive. Unless clearly usedotherwise, expressions in a singular form include a meaning of a pluralform. In the present description, an expression such as “comprising” or“including” is intended to designate a characteristic, a number, a step,an operation, an element, a part or combinations thereof, and shall notbe construed to preclude any presence or possibility of one or moreother characteristics, numbers, steps, operations, elements, parts orcombinations thereof.

Referring to the drawings, FIG. 1 illustrates a fiber with a master andslave connected to opposite ends to determine the length of the fiber,according to an exemplary embodiment.

According to an exemplary embodiment, the two ends attached to the fibermay be made up of two units. One may function in a master mode and theother may function in a slave mode. The terms master mode and slave modeare merely used as exemplary embodiments and are interchangeable withmain mode/remote mode and other terminology well known to one ofordinary skill in the art. The master unit may be attached to one end ofthe fiber. The slave may be attached to the opposite end of the fiber.

To measure the length of the fiber, the master unit sends a light pulseor a set of light pulses, as shown by the arrows in FIG. 1, with a timestamp encoded in the pulse train, according to exemplary embodiments.The slave unit receives the light pulse or the set of light pulses withthe encoded time stamp and rebroadcasts the pulse/pulses to the master.

If using the single light pulse method, the master calculates the timeit took from firing the pulse to receiving the pulse back from theslave, thereby calculating the round-trip time.

If using the train of light pulses with the time stamp of the masterencoded in the pulse train, the master subtracts the time stamp from theit's current time reference, thereby again calculating the totalround-trip time.

Accordingly, the length of the fiber can be calculated using thefollowing formula for calculating the time of flight in one direction:Time of Flight in one direction=(Calculated Round−Trip-Time inseconds)/2.

Thus, the length of the fiber can be calculated using the time of flightcalculation using the above formula.

The conversion to meters is based on the Fiber Index of Refraction.Accordingly, using the above describes exemplary embodiments, anaccurate length measurement of the fiber can be achieved.

FIG. 2 illustrates a fiber going through a process of tracing a completespan and event detection, according to an exemplary embodiment.

No OTDR today can shoot the complete span of a fiber. Therefore anaccurate trace cannot be created and all events cannot be detected. Withtoday's OTDRs, one can shoot from either side of the span; however,without having some known reference point, you cannot attach the tracesfrom either end of the span into a single, coherent, accurate trace ofthe complete span of the fiber.

According to an exemplary embodiment described in FIG. 1, the Length canbe accurately measured as described above. Therefore, a known referencepoint can be inferred by using the mid-point of the span as the knownreference point.

For example, if the span is 200 km long, then the mid-point is exactly100 km from either end. The Master unit sets its pulse width (PW) andmeasurement range to 0.5*measure length+25%((the 25% being added asbuffer). This means it sets the appropriate PW to shoot 120 km andchooses the appropriate range to measure 120 km, according to theexemplary embodiment.

Accordingly, after knowing the known reference point and performing thecalculation, the Master unit shoots the fiber and collects the trace. Atsome point beyond 120 km, the trace enters the noise floor. However, thelength of interest is the first 100 km which has been clearly capturedand therefore, accurate event detection can be performed on it.

In the same manner, the Slave unit shoots the fiber and collects thetrace. At some point beyond 120 km, the trace enters the noise floor.However, the length of interest is the first 100 km which has beenclearly captured and therefore, accurate event detection can beperformed on it.

In the exemplary embodiment of FIG. 2, if A is considered the master endof the fiber, the trace captured on the Master unit is cropped to removeanything occurring after the midpoint X.

In the same manner, the trace captured on the Slave unit B is cropped toremove anything occurring after the midpoint X. The remaining trace ofthe slave end B-X is then inverted. Specifically, by taking themeasurement loss information, the slope of the trace is inverted, asshown in FIG. 2.

Now a complete and accurate trace of the span is provided with completeevent analysis. Using the same analysis, the complete loss can also bedetermined.

FIG. 3 is a flowchart describing a method of determining the length of afiber, according to another exemplary embodiment.

The method first includes sending a light pulse or a set of light pulsesfrom the Master Unit at one end of the fiber to the slave unit at theopposite end of the fiber 301. Following that the master unit receivesthe light pulse or the set of light pulses back from the slave unit 302.

Step 303 incorporated calculating the total round-trip travel time ofthe light pulse or the set of light pulses. Finally, step 304incorporates calculating the length of the fiber using the calculatedtotal round-trip travel time, according to an exemplary embodiment.

The above described method for measuring length of the fiber is used bythe T400 and C840 optical loss test sets (OLTS) and the C850 OLTS/OTDRfor length measurement.

FIG. 4 is a flowchart describing a method of tracing a complete span andevent detection in a fiber, according to another exemplary embodiment.

The method first includes Inferring a known reference point based on themeasured length of the fiber 401. Following that a pulse width andmeasurement range is set based on the inferred reference point 402.

Step 403 incorporated shooting a fiber from the master unit to collecttrace past the inferred reference point. Step 404 incorporates shootinga fiber from the slave unit to collect trace past the inferred referencepoint.

In step 405, traces captured by both the master unit and the Slave Unitare cropped past the inferred reference point. In step 406, by takingthe measurement loss information, the slope of the trace of the slaveunit is inverted. Finally, step 407 incorporates combining the traces ofthe master unit and the slave unit to achieve a complete and accuratetrace of the fiber, according to an exemplary embodiment, as describedwith reference to FIG. 2 above.

FIG. 5 illustrates a fiber with a master and slave connected to oppositeends to determine the length of the fiber while incorporating the errorsin the time of flights of light pulses, according to art exemplaryembodiment.

As depicted in FIG. 5, the error in the time of flight for a light pulsesent from A to B will be Ea1+Eb2. Similarly, the error in the time offlight from B to A will be Eb1+Ea2.

Accordingly, the total time of flight from A to B incorporating theerrors can be calculated by the following formula:Time of Flight 1=Time of Flight A−B+Ea1+Eb2

Similarly, the total time of flight from B to A incorporating the errorscan be calculated by the following formula:Time of Flight 2=Time of Flight B−A+Ea2+Eb1

As the time of flight from A to B is the same as the time of flight B toA, the error term can be calculated using the following formula:Time of flight 1−Time of flight 2=(Ea1+Eb2)−(Eb1+Ea2)=Eab

Furthermore, the following formula uses the error term to provides uswith the average time of flight:Average time of flight=(Time of flight 1+Time of flight 2−Eab)/2

FIG. 6 illustrates a functional block diagram of an embodiment of anapparatus which determines a length of a fiber, traces a complete spanof a fiber, and performs event detection in a fiber, according to anexemplary embodiment.

The apparatus 601 includes a memory 603, a processor 602, and anOptics/Electronics unit 604, according to an exemplary embodiment. Anexample of a processor is an ARM Xscale 806 Mhz processor. An example ofa memory is an 8 Gbit NAND flash memory. Accordingly, the memory maystore a program code/operating software which in-turn instructs theprocessor 602/Optics/Electronics 604 to measure the complete spandistance, which cannot be simply estimated, measure the complete lossacross span, perform event detection and map across complete span toacquire a trace showing complete span of the fiber, as described inFIGS. 1-5 above. The program code/operating software can also be storedon a non-transitory computer readable medium.

The different exemplary embodiments described above provide a moreaccurate trace and event profile compared to a single side shot with ahigh power OTDR. They further allow use of shorter PW than required toshoot a complete span and use a shorter range than required to shoot thecomplete span thereby providing increased resolution. Furthermore, theyallow use of a less dynamic range OTDR providing a more cost effectiveand practical solution.

Although certain benefits of the described embodiments are listed above,the benefits are not limited thereto.

As mentioned above, the embodiments described above are merely exemplaryand the general inventive concept should not be limited thereto. Whilethis specification contains many features, the features should not beconstrued as limitations on the scope of the disclosure or the appendedclaims. Certain features described in the context of separateembodiments can also be implemented in combination. Conversely, variousfeatures described in the context of a single embodiment can also beimplemented in multiple embodiments separately or in any suitablesub-combination.

The invention claimed is:
 1. A method of tracing a complete span of afiber, the method comprising: determining a reference point along anend-to-end length of the fiber, wherein the reference point is specifiedat a location between the ends of the fiber along the end-to-end lengthof the fiber; setting a pulse width and a measurement range to begreater than a distance from each end of the fiber to the referencepoint; shooting a fiber from a master unit, attached to one end of thefiber, to collect a trace past the reference point; shooting the fiberfrom a slave unit, attached to an opposite end of the fiber, to collecta trace past the reference point; cropping the trace collected by themaster unit; cropping the trace collected by the slave unit; inverting aslope of the trace collected by the slave unit; and combining the tracecollected by the master unit and the trace collected by the slave unit.2. The method of claim 1, further comprising measuring an end-to-endlength of the fiber.
 3. The method of claim 1, wherein the setting thepulse width and the measurement range comprises setting the pulse widthand the measurement range to be greater than half the measured length ofthe fiber.
 4. The method of claim 1, wherein the reference point is at amidpoint of the fiber.
 5. The method of claim 1, wherein setting thepulse width and the measurement range comprises setting the pulse widthand the measurement range of the master unit and the slave unit, themaster unit and the slave unit respectively connected to the opposingends of the fiber, wherein the master unit is configured to collect thetrace from the one end of the fiber to a first point beyond thereference point in a first direction along the end-to-end length of thefiber, and wherein the slave unit is configured to collect the tracefrom the opposite end of the fiber to a second point beyond thereference point in a second opposite direction along the end-to-endlength of the fiber.
 6. The method of claim 5, wherein the first pointbeyond the reference point is determined by adding a buffer length tothe reference point in the first direction, and wherein the second pointbeyond the reference point is determined by adding a buffer length tothe reference point in the second opposite direction.
 7. A method oftracing a complete span of a fiber, the method comprising: determining areference point along an end-to-end length of the fiber, wherein thereference point is specified at a location between the ends of the fiberalong the end-to-end length of the fiber; setting a pulse width and themeasurement range of the master unit and the slave unit, the master unitand the slave unit respectively connected to the opposing ends of thefiber, wherein the master unit is configured to collect trace data fromthe one end of the fiber to a first point beyond the reference point ina first direction along the end-to-end length of the fiber, and whereinthe slave unit is configured to collect trace data from the opposite endof the fiber to second point beyond the reference point in a secondopposite direction along the end-to-end length of the fiber; shooting afiber from the master unit to collect trace past the reference point;shooting the fiber from the slave unit to collect trace past thereference point; cropping the trace data collected by the master unit;cropping the trace data collected by the slave unit; inverting a slopeof the trace data collected by the slave unit; and combining the tracedata collected by the master unit and the trace data collected by theslave unit.
 8. The method of claim 7, further comprising measuring anend-to-end length of the fiber.
 9. The method of claim 7, wherein thesetting the pulse width and the measurement range comprises setting thepulse width and the measurement range to be greater than half themeasured length of the fiber.
 10. The method of claim 7, wherein thereference point is at a midpoint of the fiber.
 11. The method of claim7, wherein the first point beyond the reference point is determined byadding a buffer length to the reference point in the first direction,and wherein the second point beyond the reference point is determined byadding a buffer length to the reference point in the second oppositedirection.